Devoted Knight (3.5e Feat)

From D&D Wiki

Your faithful service to your patron deity involves both a strong focus on melee fighting, and spell-casting.

Prerequisite: Divine spell-casting, Smite Evil.

Benefit: If you have levels in paladin and another divine spell-casting class, those levels stack for the purpose of determining both spells known and per day. This feat does not allow more spells known or per day beyond what your total class levels would normally allow.
In addition, you can multiclass freely between the paladin and the original divine class. You must still remain lawful good in order to retain your paladin abilities and to take additional paladin levels.

Normal: Different class levels do not stack for the purpose of determining spells known or per day.